The present invention relates to locks for blowout preventer rams.
In ram blowout preventers, each closure of the ram causes a certain amount of wear of the ram sealing elements which move into the borehole of the preventer for sealing contact with a pipe or other object, such as another ram. During succeeding closures of the rams, the effectiveness of the seal was reduced when the ram was locked in sealing position due to such wear.
Certain prior art blowout preventer ram locks, such as in U.S. Pat. Nos. 3,242,826 and Re27,294, used snap rings or collets mounted with a ram piston for locking. When the piston reached a predetermined locking position defined by a groove in the ram piston cylinder, the snap ring moved into the groove to lock the ram and piston in place. However, with this structure, only one locking position of the ram, as defined by the relative position of the snap ring and groove, was obtained. Change of the locking position to compensate for sealing element wear required adjustment of the relative positions of the locking elements, requiring undesirable disassembly of the blowout preventer cylinders for such adjustments to be made.
Other blowout preventer ram locks, such as in U.S. Pat. No. 3,208,357, used a taper locking pin which moved into locking position behind the ram piston once the ram had been moved into sealing position. However, extra hydraulic operating and control lines, separate and distinct from those for causing ram piston movement, were required, increasing the complexity of the control system for those types of ram locks.
In the co-pending application referred to above, these shortcomings have been for the most part overcome. However, locking action in such co-pending application was based on frictional engagement of locking rings in locking position. For high loads, however, this frictional engagement could be subject to slippage. In certain instances, unlocking of the frictionally engaging locking structure could cause difficulties. Also, dirt or particles in the operating fluid could cause galling of the frictionally engaging locking surfaces.